1. Field of the Invention
The present invention relates to a digital TV receiver, and more particularly, to an apparatus capable of judging whether the carrier frequency is locked in the digital TV receiver in VSB system and method for the same.
2. Description of the Related Art
Generally, VSB (vestigial sideband) system of Grand Alliance, employed as Korean and the USA digital TV (e.g., a HDTV) transmission standards, largely attenuates one sideband signal of two sideband signals around carrier, and modulates only the rest. In other words, the transmission system choosing only a spectrum of channels to modulate into a pass band signal is one of systems that effectively use band area.
During VSB modulation, if a DC spectrum of the base band is moved to the pass band, the DC spectrum of the base band is changed into a tone spectrum called a pilot signal. When VSB is modulated for a TV receptor to exactly copy the signals at a broadcasting station, the pilot signal is added and sent into air.
Hundreds KHz frequency offset and phase jitters are generated by a tuner or a RF generator when the TV receptor receives the VSB signals, and exact data recovery is carried out by reducing frequency offset and phase jitters to minimum. In this case, acquisition and tracking for reducing the frequency offset and phase jitters to the minimum are called carrier recovery, and the carrier recovery is carried out using the pilot signals.
However, because the pilot is reduced in a signal received in a channel environment having many reflected waves, such as an environment of a city center, use of the pilot at the carrier recoverer is limited. That is, there can be a case the data component is attenuated depending on a DTV reception channel environment, a case a frequency component in the pilot is attenuated, and, in the worst case, there can be a case the frequency component in the pilot is attenuated fully, such that no pilot component exhibits.
In a conventional VSB system digital TV, the carrier recovery is carried out using pilot components included in received signals, and whether the frequency is locked or not is determined using real number value. In this case, when carrier recovery is completely carried out, pilot exactly comes down to DC. In this case, the DC value has a value of either a positive number or a negative number. If the carrier recovery is not completely carried out because the pilot component is attenuated or not exhibited at all, the DC value keeps changing to the positive number or the negative number. In this case, the accumulated value is less than set value, and thus the carrier recovery is not exactly carried out. In the case the pilot component is reduced or fails to exhibit, accurate carrier recovery can fails. Moreover, the phase jitter, influenced from noise, becomes heavy following the reduction of the pilot.